The pH dependence of redox properties, spectroscopic
features and CO binding kinetics for the chelated
protohemin-6(7)-L-histidine methyl ester (heme-H)
and the chelated protohemin-6(7)-glycyl-L-histidine
methyl ester (heme-GH) systems has been investigated
between pH 2.0 and 12.0. The two heme systems appear
to be modulated by four protonating groups, tentatively
identified as coordinated H2O, one of heme’s propionates,
Ne of the coordinating imidazole, and the carboxylate
of the histidine residue upon hydrolysis of the
methyl ester group (in acid medium). The pKa values are
different for the two hemes, thus reflecting structural
differences. In particular, the different strain at the
Fe–Ne bond, related to the different length of the coordinating
arm, results in a dramatic alteration of the
bond strength, which is much smaller in heme-H than in
heme-GH. It leads to a variation in the variation of the
pKa for the protonation of the Ne of the axial imidazole
as well as in the proton-linked behavior of the other
protonating groups, envisaging a cross-talk communication
mechanism among different groups of the heme,
which can be operative and relevant also in the presence
of the protein matrix.

The pH dependence of redox properties, spectroscopic
features and CO binding kinetics for the chelated
protohemin-6(7)-L-histidine methyl ester (heme-H)
and the chelated protohemin-6(7)-glycyl-L-histidine
methyl ester (heme-GH) systems has been investigated
between pH 2.0 and 12.0. The two heme systems appear
to be modulated by four protonating groups, tentatively
identified as coordinated H2O, one of heme’s propionates,
Ne of the coordinating imidazole, and the carboxylate
of the histidine residue upon hydrolysis of the
methyl ester group (in acid medium). The pKa values are
different for the two hemes, thus reflecting structural
differences. In particular, the different strain at the
Fe–Ne bond, related to the different length of the coordinating
arm, results in a dramatic alteration of the
bond strength, which is much smaller in heme-H than in
heme-GH. It leads to a variation in the variation of the
pKa for the protonation of the Ne of the axial imidazole
as well as in the proton-linked behavior of the other
protonating groups, envisaging a cross-talk communication
mechanism among different groups of the heme,
which can be operative and relevant also in the presence
of the protein matrix.